Abstract Mice with a skeletal muscle (SkM)-specific decrease in the small 12 kDa FK506 binding protein, FKBP12, (FKD mice) display improved endurance, insulin-mediated glucose clearance, and bone mineral density, as well as decreased body fat and resistance to weight gain on a high fat diet. Low doses of rapamycin and SLF (synthetic ligand for FKBP12) that displace FKBP12 from its binding partners mimic the effects of FKBP12 deficiency in SkM, suggesting these drugs have potential as interventions to improve muscle function and metabolism. The primary target of FKBP12 in SkM is the sarcoplasmic reticulum (SR) Ca2+ release channel, RyR1, which controls the release of Ca2+ from intracellular stores during excitation-contraction coupling (ECC). Partial removal of FKBP12 from RyR1 (genetically or by treatment with low doses of rapamycin or SLF) increases both the amplitude of the myoplasmic Ca2+ transient and Ca2+ influx into the muscle fiber during repetitive stimulation. Both enhanced SR Ca2+ release and increased Ca2+ influx associated with partial removal of FKBP12 from RyR1 are blocked by inhibitors of calmodulin-dependent protein kinase II (CaMKII) and store-operated Ca2+ entry (SOCE). However, the mechanisms by which increases in Ca2+ release and influx result in improved muscle function and metabolism remain unknown. We hypothesize the existence of a tunable feedback loop that functionally couples ECC, SOCE, and mitochondrial Ca2+ uptake to modulate muscle function and metabolism. The specific aims of this application are to: A1. Define the roles of FKBP12 and RyR1 phosphorylation in regulating the amplitude of the Ca2+ transient during repetitive stimulation and improving SkM performance and metabolism. 2. Define the feedback loop that enhances Ca2+ store refilling and ATP production to sustain the improved muscle performance and metabolism in FKBP12 deficient mice. 3. Evaluate the therapeutic potential of SLF to improve muscle function and metabolism. Our long-term goal is to develop interventions to improve muscle function in people who cannot perform strenuous exercise due to age, muscle disease, obesity and/or have type II diabetes.